Hi,
Here is an update on the Gravity Probe B mission as of Nov 26, 2004.
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GRAVITY PROBE B MISSION UPDATE FOR 26 NOVEMBER 2004
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On the eve of the 2004 Thanksgiving holiday weekend-mission week #32--the GP-B spacecraft is in good health, with all subsystems performing well. We have now been collecting data for three months. Data collection is proceeding smoothly, and the quality of the data is excellent. The spacecraft continues to fly drag-free around gyro #3, maintaining a constant roll rate of 0.7742 rpm (77.5 seconds per revolution.) The temperature inside the Dewar remains steady at just under 1.82 kelvin.
Last week's update prompted questions about seasonal temperature variations in the GP-B spacecraft and the effects of these variations on the Dewar and the spacecraft's various subsystems. Seasons on the Earth are due to the 23-degree tilt in the Earth's axis relative to the orbital plane in which we circle the Sun. This tilt results in varying intensity and duration of sunlight at different latitudes during the four seasons of the year. For example, sunlight shines more directly, and for more hours (longer days) on locations in the Northern Hemisphere in the summer months, versus the winter months, when the days are shorter and the sun shines more obliquely on the Northern Hemisphere. This situation is reversed in the Southern Hemisphere.
The GP-B spacecraft experiences four "seasons" as well, but they are not directly due to the tilt in Earth's axis. Rather, the spacecraft alternates between hot and cold seasons through the course of a year due to the changing relationship of the spacecraft's orbital plane with respect to the Sun, as the Earth and spacecraft revolve around the Sun. The changing geometry between the Sun and the spacecraft's orbital plane affects the amount of sunlight shining on the front, side, or back of the spacecraft. Because the spacecraft is in a polar orbit, with the plane of its orbit continually aligned with the guide star, there are two six-week periods when the Sun shines broadside on the spacecraft throughout all of its daily orbits. These are the spacecraft's "hot" seasons.
The first of these "hot" seasons occurred shortly after launch, between mid-May and the end of June. The second "hot" season began last week and will last through the end of December. Likewise, another 6-week "hot" season will begin in the middle of May, next year. At all other times of the year, the spacecraft is in its "cold seasons," moving behind the Earth--out of sunlight and into the cold void of space--for a part of each orbit. The time when the spacecraft moves behind the Earth is called the orbital eclipse period, and the maximum eclipse period occurs in August-September and again in February-March. At these times, the spacecraft is out of sunlight for approximately half of each orbit. Two diagrams accompanying this week's highlights on our GP-B Web site-one depicting Earth's seasons and the other depicting the spacecraft's seasons-will help you visualize the geometric relationships between the Sun, Earth, and spacecraft.
One effect of the spacecraft's "hot" seasons is the warming of the Dewar shell, which we have been reporting in the past two weekly updates. In order to maintain the cryogenic temperature and pressure conditions inside the Dewar requisite for superconductivity in the gyro rotors and SQUID readouts, the flow of helium boiling out of the Dewar increases as the Dewar shell warms up. In turn, the Attitude and Translation Control system (ATC) null-dumps (uniformly vents) this excess helium through the micro thruster system in order to maintain the spacecraft's proper attitude and pointing direction.
Another effect of the "hot" season is a temperature rise in the Forward Equipment Enclosure (FEE) at the front end of the spacecraft. Among other things, the FEE includes the telescope and SQUID readout (SRE) control electronics. In the spacecraft's "cold" seasons, the heat generated by these electronics boards is radiated to the outer wall of the FEE and then into space when the spacecraft cools off during the eclipsed portion of each orbit. However, in the "hot" seasons, when the sun is shining broadside on the spacecraft throughout each orbit, there is no eclipse period to cool off the FEE, so cooling of this equipment must be accomplished by increasing the set points of heaters in the FEE, which creates a temperature differential for radiating heat away from these important circuits. For the past two weeks, as the spacecraft has been entering a "hot" season, the team has been monitoring and adjusting these temperature controls in order to maintain the proper temperature in the FEE. As orbital eclipsing of the spacecraft returns towards the end of December, the team will re-adjust these thermal controls for the oncoming "cold" season.
One nice side effect of the "hot" seasons is that the sunlight, shining broadside on the spacecraft, makes it more prominent to see from various locations on Earth in clear skies. The spacecraft is best viewed at times when it passes nearly overhead to your location, in the pre-dawn or evening twilight. A typical GP-B pass lasts about 15 minutes from horizon to horizon. From a viewing location that is devoid of city lights, you can see the GP-B spacecraft with your naked eye; in locations with more ambient light, binoculars may be helpful. Our GP-B Web site contains links to satellite tracking Web pages for computers and PDAs (Palm Pilot, Blackberry, etc.):
http://einstein.stanford.edu/content/faqs/faqs.html#tracking.
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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.eduBob Kahn
Public Affairs Coordinator
Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,
LelandJ
